To use the smallest number of identical square tiles, the side length of each tile must be as large as possible.
The side length of the square tile must perfectly divide both the length (24 units) and the width (18 units) of the floor.
Therefore, the side length must be the Greatest Common Divisor (GCD) of 18 and 24.
So, the largest possible identical square tile has a side length of 6 units.
The number of tiles needed is determined by how many tiles fit along each dimension of the floor.
Total number of tiles = (Tiles along width) $ \times $ (Tiles along length)
Total number of tiles = $ 3 \times 4 = 12 $ tiles.
Alternatively, using area:
The smallest number of identical square tiles required is 12.